Oxycytochrome P-450: its breakdown to superoxide for the formation of hydrogen peroxide.

Four different experimental studies are described which were designed to evaluate the role of oxycytochrome P-450 in the formation of superoxide anions and hydrogen peroxide. The use of lipophilic copper chelates with superoxide dismutase like activity revealed that the primary site of interaction of these agents is related to the inhibition of the flavoprotein. NADPH-cytochrome P-450 reductase. Measurements of the proton assisted nucleophilic displacement of superoxide from oxycytochrome P-450 by high concentrations of sodium azide indicated an increase in the rate of hydrogen peroxide formation concomitant with the inhibition of the N-demethylation of ethylmorphine. Studies on the effect of NADH on the rate of hydrogen peroxide formation during NADPH oxidation by liver microsomes failed to reveal a stimulatory or synergistic effect in a manner analogous to results obtained during the cytochrome P-450 dependent oxidation of substrates such as ethylmorphine. These results suggest that hydrogen peroxide formation may not require the reduction of oxycytochrome P-450 to peroxycytochrome P-450. Measurements of the reduction of succinylated cytochrome c using purified cytochrome P-450 and the flavoprotein, NADPH-cytochrome P-450 reductase, directly demonstrate the formation of superoxide anions. It is concluded that oxycytochrome P-450 may decompose to generate hydrogen peroxide.     

Inhibition of cytochrome P-450-linked monooxygenase systems by naphthoquinones

Several naphthoquinones, except 2-hydroxy-1,4-naphthoquinone, were found to inhibit microsomal cytochrome P-450-linked monooxygenase activities in rabbit liver and human placenta. In particular, 5-hydroxy-1,4-naphthoquinone inhibited placental estrogen biosynthesis more effectively than it did hepatic drug oxidation reactions. There was little contribution by superoxide radicals to these enzyme inhibitions by naphthoquinones. Spectrophotometric studies revealed that naphthoquinones bind to the cytochrome P-450 component of the monooxygenase complex in both microsomal systems, suggesting that the inhibition is caused by direct interaction of these compounds with the heme.     

Superoxide dismutase-like activities of copper(II) complexes tested in serum

The superoxide dismutase-like activities of a series of coordination complexes of copper were evaluated and compared to the activities of bovine erythrocyte superoxide dismutase (superoxide: superoxide oxidoreductase, EC 1.15.1.1) in serum using the nitroblue tetrazolium chloride (NBT)-reduction assay and electron paramagnetic resonance (EPR) spectroscopy. A 40% inhibition was observed for the initial rate of the NBT reduction by superoxide dismutase in serum, but more than 40% inhibition was achieved with CuSO4, Cu(II)-dimethylglyoxime, Cu(II)-3,8-dimethyl-4,7-diazadeca-3,7-dienediamide, Cu2[N,N'-(2-(O-hydroxy-benzhydrylidene)amino)ethyl]2-1,2-ethane dia mine), Cu(II)-(diisopropylsalicylate)2, Cu(II)-(p-bromo-benzoate)2, Cu(II)-(nicotinate)2 and Cu(II)-(1,2-diamino-2-methylpropane)2. The electron paramagnetic resonance technique of spin trapping was used to detect the formation of superoxide (O2-.) and other free radicals in the xanthine-xanthine oxidase system under a variety of conditions. Addition of the spin trapping agent 5,5-dimethylpyrroline 1-oxide (DMPO) to the xanthine-xanthine oxidase system in fetal bovine serum produced the O2-.-spin adduct of DMPO (herein referred to as superoxide spin adduct, DMPO-OOH) as the well known short-lived nitroxyl whose characteristic EPR spectrum was recorded before its rapid decay to undetectable levels. The hydroxyl radical (HO.) adduct of the spin trap DMPO (herein referred to as DMPO-OH) was detected to a very small extent. When CuSO4, or the test complexes of copper, were added to the xanthine-xanthine oxidase system in serum containing the spin trap, the yield of DMPO-OOH was negligible. In addition to their superoxide dismutase-like activity, CuSO4 and the copper complexes also behaved as Fenton-type catalysts as seen by the accumulation of varying amounts of the hydroxyl spin adduct DMPO-OH. Both the Fenton-type catalysis and the superoxide dismutase-like action of these compounds were lost when a chelator such as EDTA was included in the xanthine-xanthine oxidase incubation mixture. Addition of superoxide dismutase instead of the copper compounds to this enzyme system abolished the formation of superoxide adduct DMPO-OOH, and no hydroxyl adduct DMPO-OH was detected. This effect of superoxide dismutase remained unaltered by EDTA.     

Mechanisms of hydroxyl radical formation and ethanol oxidation by ethanol-inducible and other forms of rabbit liver microsomal cytochromes P-450

The hydroxyl radical-mediated oxidation of 5,5-dimethyl-1-pyrroline N-oxide, benzene, ketomethiolbutyric acid, deoxyribose, and ethanol, as well as superoxide anion and hydrogen peroxide formation was quantitated in reconstituted membrane vesicle systems containing purified rabbit liver microsomal NADPH-cytochrome P-450 reductase and cytochromes P-450 LM2, P-450 LMeb , or P-450 LM4, and in vesicle systems devoid of cytochrome P-450. The presence of cytochrome P-450 in the membranes resulted in 4-8-fold higher rates of O-2, H2O2, and hydroxyl radical production, indicating that the oxycytochrome P-450 complex constitutes the major source for superoxide anions liberated in the system, giving as a consequence hydrogen peroxide and also, subsequently, hydroxyl radicals formed in an iron-catalyzed Haber-Weiss reaction. Depletion of contaminating iron in the incubation systems resulted in small or negligible rates of cytochrome P-450-dependent ethanol oxidation. However, small amounts (1 microM) of chelated iron (e.g. Fe3+-EDTA) enhanced ethanol oxidation specifically when membranes containing the ethanol and benzene-inducible form of cytochrome P-450 (cytochrome P-450 LMeb ) were used. Introduction of the Fe-EDTA complex into P-450 LMeb -containing incubation systems caused a decrease in hydrogen peroxide formation and a concomitant 6-fold increase in acetaldehyde production; consequently, the rate of NADPH consumption was not affected. In iron-depleted systems containing cytochrome P-450 LM2 or cytochrome P-450 LMeb , an appropriate stoichiometry was attained between the NADPH consumed and the sum of hydrogen peroxide and acetaldehyde produced. Horseradish peroxidase and scavengers of hydroxyl radicals inhibited the cytochrome P-450 LMeb -dependent ethanol oxidation both in the presence and in the absence of Fe-EDTA. The results are not consistent with a specific mechanism for cytochrome P-450-dependent ethanol oxidation and indicate that hydroxyl radicals, formed in an iron-catalyzed Haber-Weiss reaction and in a Fenton reaction, constitute the active oxygen species. Cytochrome P-450-dependent ethanol oxidation under in vivo conditions would, according to this concept, require the presence of non-heme iron and endogenous iron chelators.     

Comparative study of effects of the tyrosine-copper(II) complex on xenobiotic hydroxylation and lipid peroxidation]

It has been found that NADPH-dependent hydroxylation of dimethylaniline, aniline, p- and o-nitroanisol and lipid peroxidation is inhibited by the tyrosine-copper (II) complex (low molecular weight analog of superoxide dismutase), which is indicative of a possibility of superoxide radicals formation in these reactions. The inhibition of the above-mentioned reactions with Tyr2-Cu2+ is less pronounced or absent, if cumole hydroperoxide is used as cosubstrate instead of NADPH. Differences in the Tyr2-Cu2+ complex effects on the cumule hydroperoxide-dependent xenobiotics hydroxylation and lipid peroxidation catalyzed by various forms of cytochrome P-450, e. g. microsomal, soluble and incorporated into liposomes, have been found. The data obtained suggest that the efficiency of the inhibitory effect of the Tyr2-Cu2+ complex depends on the type of cosubstrates (NADPH, cumole hydroperoxide) and substrates used as well as on the form of cytochrome P-450.     

Inhibition of the microsomal stearoyl coenyme A desaturation by divalent copper and its chelates

Divalent copper and copper complexes of tyrosine, histidine and lysine inhibited at low concentrations the stearoyl-CoA desaturation reaction in both chicken liver microsomes and in a purified system consisting of chicken liver delta 9 terminal desaturase, cytochrome b5, ascorbate and liposome. Although the copper chelates lowered the steady-state level of ferrocytochrome b5 by 20%, and partially inhibited the NADH-ferricyanide reductase activity, the availability of the ferrocytochrome b5 during the time course of desaturation was not affected, indicating that the site of inhibition of desaturation was at the terminal step, i.e., on the delta 9 terminal desaturase. The presence of chalates during catalysis was essential for the observed inhibition. Based on the observation that O2 is involved in the desaturation and that there is an initial electron reduction of desaturase iron, it is plausible that the copper chelates are inhibiting by acting as superoxide scavengers.     

Relationship between the reduction of oxygen, artificial acceptors and cytochrome P-450 by NADPH--cytochrome c reductase.

The interaction of NADPH--cytochrome c reductase with oxygen, artificial acceptors and cytochrome P-450 was studied. The generation of superoxide anion radicals (O2-.) from the oxidation of adrenaline to adrenochrome catalysed by NADPH--cytochrome c reductase proceeds independently of the interaction of the enzyme with the artificial anaerobic acceptors cytochrome c or 2,6-dichlorophenol-indophenol. Propyl 3,4,5-trihydroxybenzoate inhibited competitively the adrenaline oxidation by isolated NADPH--cytochrome c reductase (Ki 3.2--4.7 micrometer) and inhibited non-competitively the cytochrome c reduction (Ki 92--109 micrometer). In contrast with the process of electron transfer to cytochrome c, the rate of reduction of cytochrome P-450 and the rate of oxidation of adrenaline in liver microsomal fraction are correlated. Hexobarbital increases the Vmax. of adrenaline oxidation without affecting the Km value, whereas metyrapone, a metabolic inhibitor decreases Vmax. without affecting the Km. From the results obtained, some conclusions about NADPH--cytochrome c reductase function were made.     

The Copper Complex of Captopril is not a Superoxide Dismutase Mimic. Artefacts in DMPO Spin Trapping

The effect of captopril and of its copper complex on several superoxide-dependent reactions used to detect and assay superoxide dismutase activity was studied, including pyrogallol and hematoxylin autoxidation and Nitro Blue Tetrazolium reduction. ln none of these systems were superoxide dismutase-like properties of captopril/Cu apparent. Captopril/Cu decreased the yield of DMPO-OH adducts generated by KO₂ but this effect may be due to the acceleration of the decay of the adduct by captopril/Cu.     

Hematoporphyrin photosensitization of epidermal microsomes results in destruction of cytochrome P-450 and in decreased monooxygenase activities and heme content

Epidermal microsomal cytochrome P-450 was rapidly degraded when microsomes were aerobically exposed to ultraviolet light in the presence of hematoporphyrin derivative (HPD). Destruction of microsomal cytochrome P-450 was accompanied by loss of heme content, and inhibition of catalytic activity of the monooxygenases, including aryl hydrocarbon hydroxylase and 7-ethoxycoumarin-O-deethylase. Destruction of cytochrome P-450 by photosensitized HPD was oxygen dependent. Quenchers of singlet oxygen, including 2,5 dimethylfuran, histidine, and B-carotene, largely pre- vented photodestruction of cytochrome P-450. Inhibitors of hydroxyl radical including benzoate and mannitol, protected microsomal cytochrome P-450 from destruction. Superoxide dismutase and catalase, scavengers of superoxide anion and hydrogen peroxide, respectively, had no protective effect. These results indicate that generation of singlet oxygen and hydroxyl radicals during hematoporphyrin photosensitization is associated with rapid degradation of cytochrome P-450 and heme in epidermal microsomes, and suggest a novel target for this type of tissue damage in the skin.     

On the mechanism of action of cytochrome P-450. Oxidation and reduction of the ferrous dioxygen complex of liver microsomal cytochrome P-450 by cytochrome b5

The effects of cytochrome b5 on the decay of the ferrous dioxygen complexes of P-450LM2 and P-450LM4 from rabbit liver microsomes were studied by stopped-flow spectrophotometry. The P-450 (FeIIO2) complexes accept an electron from reduced cytochrome b5 and, in a reaction not previously described, donate an electron to oxidized cytochrome b5 to give ferric P-450. A comparison with the electron-transferring properties of ferrous P-450 under anaerobic conditions allowed determination of the limiting steps of the two reactions involving the oxygenated complex. The rate of decay of the dioxygen complex was increased in all cases with b5 present; however, with oxidized b5 a large increase in the rate was observed with P-450 isozyme 4 but not with isozyme 2, whereas the opposite situation was found when reduced b5 was used. The reactions between b5 and ferrous dioxygen P-450 were not at thermodynamic equilibrium under the conditions employed. From the results obtained, a model is proposed in which the ferrous dioxygen complex decomposes rapidly into another species differing from ferric P-450 in its spectral properties and from the starting complex in its electron-transferring properties. A scheme is presented to indicate how competition among spontaneous decay, cytochrome b5 oxidation, and cytochrome b5 reduction by the ferrous O2 complex may influence substrate hydroxylation.     

Hydroxyl radical-mediated, cytochrome P-450-dependent metabolic activation of benzene in microsomes and reconstituted enzyme systems from rabbit liver

The mechanism of benzene oxygenation in liver microsomes and in reconstituted enzyme systems from rabbit liver was investigated. It was found that the NADPH-dependent transformation of benzene to water-soluble metabolites and to phenol catalyzed by cytochrome P-450 LM2 in membrane vesicles was inhibited by catalase, horseradish peroxidase, superoxide dismutase, and hydroxyl radical scavengers such as mannitol, dimethyl sulfoxide, and catechol, indicating the participation of hydrogen peroxide, superoxide anions, and hydroxyl radicals in the process. The cytochrome P-450 LM2-dependent, hydroxyl radical-mediated destruction of deoxyribose was inhibited concomitantly to the benzene oxidation. Also the microsomal benzene metabolism, which did not exhibit Michaelis-Menten kinetics, was effectively inhibited by six different hydroxyl radical scavengers. Biphenyl was formed in the reconstituted system, indicating the cytochrome P-450-dependent production of a hydroxycyclohexadienyl radical as a consequence of interactions between hydroxyl radicals and benzene. The formation of benzene metabolites covalently bound to protein was efficiently inhibited by radical scavengers but not by epoxide hydrolase. The results indicate that the microsomal cytochrome P-450-dependent oxidation of benzene is mediated by hydroxyl radicals formed in a modified Haber-Weiss reaction between hydrogen peroxide and superoxide anions and suggest that any cellular superoxide-generating system may be sufficient for the metabolic activation of benzene and structurally related compounds.     

Use of spin-labelled substrate analogs for a comparative study of microsomal cytochrome P-450 and P-448

The previously described, iodine-labeled alkylating stable nitroxyl radicals located at different distances between the N-O. group and the iodine atom were used for a comparative study of the structure of microsomal cytochromes P-450 and P-448 active centers. The radicals were shown to change the optical spectra of Fe3+ located in the active site of the enzyme that are similar to those induced by cytochrome P-450 substrates. Some differences in the type of the radicals binding to control, phenobarbital- and 3-methylcholanthrene-induced microsomes were revealed. The alkylating radical substrate analogs covalently bound to microsomal cytochrome P-450 in the vicinity of the active center, resulting in the inhibition of oxidation of type I and II substrates (e. g., aniline and naphthalene). The value of the spectral binding constant (Ks) for naphthalene in the presence of the radical covalently bound to the cytochrome P-450 active center showed a tendency to increase. Using the ESR technique, the interaction between Fe3+ and the radical localized in the active site of cytochrome P-450 was demonstrated. The contribution of Fe3+ to the relaxation of the radicals covalently bound to cytochrome P-450 was evaluated from the values of the spin label ESR spectra saturation curves at 77K. The distances between the N-O. group of these radicals and Fe3+ in the enzyme active center for the three types of microsomes were determined. The data obtained point to structural peculiarities of the active center of cytochrome P-450, depending on the microsomal type.     

The apparent inhibition of superoxide dismutase activity by quinones

It has been demonstrated that several quinones can modify the activity of bovine copper superoxide dismutase by undergoing equilibrium reactions with superoxide radicals. The extent of this apparent inhibition correlates with the one electron reduction potentials of the quinones and the equilibrium constants of the semiquinone radical/superoxide radical reactions. Various rate constants have been estimated including those for the reactions of semiquinone radicals with cytochrome c and with superoxide dismtuase. Semiquinone radicals cannot be dismutated by superoxide dismutase.     

Effects in vitro of copper and zinc on hepatic cytochrome P-450 activities

1. In previous studies we have shown that hepatic copper and zinc increases and liver microsomal cytochrome P-450 activities greatly decreases in adjuvant arthritic rats. 2. In the present paper we study if the changes in copper and zinc could be related to depression of drug microsomal activity. Thus, the effect of in vitro addition of copper or zinc to microsomal fraction upon aminopyrine N-demethylase (AND) and aniline p-hydroxylase (APH) activity was measured. 3. Both metals produced an inhibition of enzyme activity. The reduction of AND and APH activities produced by copper (ID25 = 4.7 x 10(-5)M to AND; 1.05 x 10(-5)M to APH) was greater than that obtained with zinc (ID25 = 2.26 x 10(-4)M to AND; 3.3 x 10(-4)M to APH).     

Inactivation of cytochrome P-450 by hydrogen peroxide formed in the catalytic cycle during peroxy-complex degradation

It was shown that the crucial role in the inactivation of microsomal cytochrome P-450 in reactions of hydroxylation of type I (DMA, AP, BPh, p-NA) and type II (AN) substrates belongs to H2O2 directly formed in the enzyme active center during the decomposition of the peroxy complex. Hydrogen peroxide formed via an indirect pathway during the dismutation of superoxide radicals does not play a role in the hemoprotein inactivation.     

Demethylation and denitrosation of nitrosamines by cytochrome P-450 isozymes

Metabolism of nitrosamines was studied in a reconstituted monooxygenase system composed of cytochrome P-450 isozymes purified from liver microsomes of ethanol- and phenobarbital-treated rats. The ethanol-induced isozyme (P-450et) was efficient in catalyzing the demethylation of N-nitrosodimethylamine (NDMA), with a Km of 2.4 mM and Vmax of 7.2 nmol min-1 nmol P-450(-1), but less active with N-nitrosomethylbenzylamine and N-nitrosomethylaniline. The phenobarbital-induced form (P-450b) was ineffective in NDMA metabolism but was active in catalyzing the demethylation of N-nitrosomethylaniline, with an estimated Km of 0.08 mM and a Vmax of 7.2 nmol min-1 nmol-1. P-450et also catalyzed the denitrosation of NDMA with a Km of 13.6 mM and a Vmax of 1.36 nmol min-1 nmol-1. With control liver microsomes, multiple Km values were observed for the demethylation and denitrosation of NDMA. Involvement of superoxide radicals in the metabolism of NDMA was suggested by the action of superoxide dismutase, which inhibited the denitrosation by 43 to 73% and the demethylation by 13 to 22% in different monooxygenase systems. The P-450et-dependent NDMA demethylation was strongly inhibited by 2-phenylethylamine and 3-amino-1,2,4-triazole; these compounds were previously believed not to be inhibitors of P-450-dependent reactions but were found to inhibit microsomal NDMA demethylase. The present results establish the role of P-450 in nitrosamine metabolism and help to clarify some of the previous confusion in this area of research.     

Iron and copper metabolism

Iron and copper are essential nutrients, excesses or deficiencies of which cause impaired cellular functions and eventually cell death. The metabolic fates of copper and iron are intimately related. Systemic copper deficiency generates cellular iron deficiency, which in humans results in diminished work capacity, reduced intellectual capacity, diminished growth, alterations in bone mineralization, and diminished immune response. Copper is required for the function of over 30 proteins, including superoxide dismutase, ceruloplasmin, lysyl oxidase, cytochrome c oxidase, tyrosinase and dopamine-beta-hydroxylase. Iron is similarly required in numerous essential proteins, such as the heme-containing proteins, electron transport chain and microsomal electron transport proteins, and iron-sulfur proteins and enzymes such as ribonucleotide reductase, prolyl hydroxylase phenylalanine hydroxylase, tyrosine hydroxylase and aconitase. The essentiality of iron and copper resides in their capacity to participate in one-electron exchange reactions. However, the same property that makes them essential also generates free radicals that can be seriously deleterious to cells. Thus, these seemingly paradoxical properties of iron and copper demand a concerted regulation of cellular copper and iron levels. Here we review the most salient characteristics of their homeostasis.     

Ferrous ion-mediated cytochrome P-450 degradation and lipid peroxidation in adrenal cortex mitochondria.

The relationship between the degradation reaction of cytochrome P-450 and lipid peroxidation was studied utilizing bovine adrenal cortex mitochondria. The two reactions were found to be closely correlated in terms of their response to storage of the mitochondrial preparation, stimulation by Fe2+, inhibition by EDTA and their initiation by cumene hydroperoxide. Both reactions were also found not to be inhibited by catalase, superoxide dismutase, 1,4-diazabicyclo-(2,2,2)-octane and alcohols, indicating that H2O2, superoxide, singlet oxygen and hydroxyl radicals do not participate in these reactions. Yet, diphenylamine proved to be a powerful inhibitor for both reactions, suggesting the involvement of a radical species. Cumene hydroperoxide could induce these two reactions at below 0.1 mM concentrations in the presence of molecular oxygen. The chemiluminescence observed during the Fe2+-mediated lipid peroxidation reaction which was not inhibited by either superoxide dismutase or 1,4-diazabicyclo-(2,2,2)-octane, was biphasic: one was a rapid burst; and the other was a slowly increasing emission. The latter portion of the emission of light coincided with the formation of malondialdehyde. These results indicate that in adrenal cortex mitochondria the degradation of cytochrome P-450 is closely related to lipid peroxidation.     

Effect of 2-hydroxyestradiol-17beta on NADPH-dependent electron transfer in rat liver microsomes in vitro (author's transl)]

If rat liver microsomes are incubated with NADPH and 2-hydroxyestradiol-17beta in vitro, the following is observed: 1.Inhibition of lipid peroxidation, 2.inhibition of cytochrome P-450 reduction, and 3.inhibition of cytochrome b5 reduction. Beyond this the catechole inhibits lipid peroxidation of liposomes in vitro. These phenomena can be explained by interaction of different states of oxidation of the estrogen with the NADPH-cytochrome reductase and with 0-2 radicals, which leads to terminal "uncoupling" of microsomal electron transport.     

Chemiluminescence studies on the generation of oxygen radicals from the interaction of NADPH-cytochrome P-450 reductase with iron

The ability of NADPH-cytochrome P-450 reductase to interact with iron and generate oxygen radicals was evaluated by assaying for low level chemiluminescence. The basic reaction system which contained the reductase, an NADPH-generating system, ferric-EDTA as an electron acceptor, and t-butyl hydroperoxide as the oxidant acceptor, resulted in the production of chemiluminescence. Omission of any of these components resulted in a complete loss of chemiluminescence. The light emission was completely sensitive to inhibition by glutathione and butylated hydroxytoluene, partially sensitive (about 60% decrease) to catalase and hydroxyl radical scavengers, and relatively insensitive (about 20% decrease) to superoxide dismutase. The ability of other ferric chelates to replace ferric-EDTA in catalyzing the reductase-dependent chemiluminescence was evaluated. Ferric-citrate, -ADP, -ATP, and ferric-ammonium sulfate were ineffective in promoting chemiluminescence, whereas ferric-diethylenetriaminepentaacetic acid was even more effective than ferric-EDTA. Thus, the ferric chelates, which catalyze reductase-dependent chemiluminescence, are those which are efficient electron acceptors from the reductase and were previously shown to be those capable of catalyzing hydroxyl radical production by microsomes and the reductase. It is suggested that chemiluminescence results from (a) the direct interaction of the reduced iron chelate with the hydroperoxide (Fenton-type of reaction) to generate alkoxyl and peroxyl radicals, and (b) the generation of hydroxyl radicals, which subsequently react with the hydroperoxide to generate secondary radicals. The latter, but not the former, would be sensitive to inhibition by catalase and competitive hydroxyl radical scavengers, whereas both would be sensitive to antioxidants such as butylated hydroxytoluene. Chemiluminescence appears to be a versatile tool for studying the reductase-dependent generation of oxygen radicals and for the interaction of reductase with iron.